One seventh of total energy consumption is from biomass which is the main energy resource for over 1.5 billion people in the world. Biomass energy is the only one which has both the property of fossil fuel and characteristics which mean that it can be stored, renewed and transferred. It is less restricted by natural conditions. Biomass energy can be transferred to useful thermal energy, electrical energy and the fuel as power by means of direct combustion, gasification and liquidation. High-grade combustible gas like CO, H2 and methane can be formed by the gasification of biomass. Biogas can be produced by anaerobic digestion of biomass and liquid fuel using, for example, thermalization, biochemistry, machinery and chemistry.

Energy is the material basis on which human beings rely and carry out economic development. Its reasonable development and efficient utilization relates to the future of the world. Renewable biomass energy occupies an important position and plays a decisive role in the present world energy structure. According to statistics, one seventh of the total world energy consumption is from biomass. In developing countries, the proportion of biomass in total energy consumption is even more. At present, about 1.5 billion of the world population has biomass as its main energy resource. So, how to use biomass energy, a kind of potential renewable energy resource, more efficiently and make it play an important role in the coming century. This is an issue which is being paid more and more attention.

Biomass energy is the roost renewable energy resource in the world. Like a great solar chemical industry plant, it spreads in plants all over land and water throughout the world which transfer continuously solar energy into chemical energy which is stored in the inner part of plants in the form of organic matter. So, a kind of abundant renewable energy - biomass energy is composed.

About 120 billion tons of biomass, the energy capacity of which is five times the total present energy consumption in the world, is formed each year by means of photosynthesis (see Table 1) . But only 1% of the total energy capacity has been used as energy. It can supply about 14% of total energy consumption in the world.

China is relatively rich in biomass energy resources. Five billion tons of biomass can be produced in the whole country annually, among which 700 million tons of it is from the agricultural sector. In rural energy consumption in China, biomass energy takes up about 70%.

Biomass energy is the only one that has both the property of fossil fuel and characteristics which mean that it can be stored, renewed and transferred. It is less limited by natural conditions. According to where it is found, biomass energy can be divided into forest biomass, agricultural biomass and aquatic biomass. It can be divided into one-time energy resource and two-times energy resource in the form of forming process. The former refers to various forest, agricultural crops, grass and aquatic plants that are from photosynthesis and the later refers to biomass energy that is produced indirectly, including animal manure, multiple organic waste, organic waste water, effluent sludge as well as other organic wastes formed in industry and agriculture processing. All this matter contains abundant energy.

Table 1. Annual Biomass Energy Yield from Residue in Different Areas in the World in 1987 unit: EJ (1018 J)

Area

Maize Straw

Wheat Straw

Rice Straw

Bagasse

Manure

Forest Residue

Firewood Forest

Total

Industrialized countries and areas

USA & Canada

2.95

1.93

0.13

0.19

3.08

7.66

0.92

16.86

Europe

0.61

2.39

0.04

0

4.22

4.12

0.41

11.79

Japan

0

0.02

0.24

0.01

0.30

0.41

0

0.98

Australia

0

0.29

0.02

0.19

1.36

0.35

0.02

2.23

& New Zealand

0.23

1.97

0.04

0

3.58

3.92

0.60

10.34

sum

3.8

6.6

0.5

0.4

12.5

16.5

1.9

42.2

Developing countries and areas

Latin America

0.71

0.38

0.29

3.58

7.21

1.47

2.12

15.76

Africa

0.48

0.25

0.20

0.54

5.38

0.75

3.31

10.91

China

1.23

1.75

3.43

0.48

4.81

1.27

1.34

14.31

other Asian countries

0.51

1.88

5.29

2.70

10.91

2.31

4.62

28.22

the Pacific

0

0

0

0.03

0.02

0.05

0.04

0.14

sum

2.9

4.3

9.2

7.3

28.3

5.8

11.4

69.2

total in world

6.7

10.9

9.7

7.7

40.8

22.3

13.3

111.4

The accumulation of energy varies with type of plants. Different plants consist of different energy and this depends on the element composition of the plant. The element composition of wood is as follows: C: 49.5%, H: 6.5%, 0: 43%, N: 1% and water content: 15% - 20%. The thermal outputs of woody fuels vary with the difference of tree variety and different parts of the tree. The thermal value is 16700 - 18800 kJ/kg. The element composition of agricultural residue is as follows: C 40% - 46%, H 4% - 6%, 0 43% - 50%, N 0.6% - 1.1%, S 0.1% - 0.2%, P 1.5% - 2.5%.

Generally, the residue which contains more carbon has a higher thermal output. The thermal value of residue fuels is usually 14200 - 15500 kJ/kg which is less than that of woody fuels.

The biomass energy resource has many advantages such as vaster resource capacity, lower price, less sulphur composition, less ash content and the feature of renewability. On the other hand, it also has some unfavorable aspects such as higher water content, lower unit thermal output, large volume, decentralized resource and unsuitable for collection, storage and transportation. But these disadvantages can be overcome. As long as suitable measures are adopted, taking into account local conditions, using available science and technology, selecting a reasonable technical program, adopting advanced techniques, developing new energy conversion methods and paying attention to the energy utilization efficiency and economy of biological system, then biomass resources can be used efficiently.

In biomass conversion systems, each linkage can bring benefit to human beings. It has the characteristics of a good all round system. The cloths, food, residence and travel of human beings rely on it. Its waste material can be used as energy and the final residue can be returned to fields as humus, especially the photosynthesis of green plants can improve the worsening environment resulting from over-emitted carbon dioxide, which is the feature that other energy resources cannot reach.

As an energy source, biomass benefits to alleviate the contradictions between increasing demand and declining sources of conventional energy. The utilization of biomass also can improve the polluted and worsening environment. Thus, more and more attention is being paid to it day by day.

A few centuries ago, biomass was the most important energy resource. At the present day with advanced science and technology, the energy supplied by biomass is still more than that of both hydraulic energy and nuclear energy. 14% of energy in Sweden is from biomass. The proportion of biomass in the energy economy of Holland has been increased to 12%. In USA about 9 billion Watts of electricity are produced from biomass. In developing countries, about 35% of energy is from biomass. Since 1977, Brazil has begun to make ethanol from sugar cane and maize, and the annual output reaches 12 billion liters.

By means of transferring, biomass can be converted to useful thermal energy, electricity and fuels for power. The main converting methods are: direct combustion, gasification and liquidation of biomass.

3.1 Direct combustion

Combustion is the most common and traditional way to produce heat from biomass. In developing countries, the thermal efficiency of direct biomass combustion is 10% - 15% generally. After transformation the thermal efficiency of stoves in rural China are about 30%, and the best can reach up to 50%. The stove is composed of a combustion chamber, fire fencing ring, smoke circulation passage, chimney, stove door, grate and air inlet. The key design points are to increase the intensity of thermal radiation and reflection in the combustion chamber and reduce the loss of complete combustion in the inner stove and the thermal loss of smoke.

Some advanced European countries adopt high-efficiency combustion equipment such as sulphurized-bed combustion equipment. In the equipment, wood is cut into small pieces which then cross the sulphurised bed in a very short time. After combustion, the incompletely burned wood pieces are returned to the sulphurised bed from the smoke exhaust system. The commercialized small- and middle-sized boilers developed by these countries take wood and residues as fuel. Their efficiencies can reach 50% - 60%. In Holland, there are about 1.75 million sets of wood stoves with specifications of 5 - 20 kW and 600, 000 wood fireplaces for domestic heating and hot water supply. Their thermal efficiencies can reach to over 50%. The thermal efficiency of fixed bed model boilers burning grass and manufactured by England and Denmark are 60%.

3.2 Gasification of biomass

3.2.1 Pyrolysing gasification of biomass is one of the optimum biomass utilization technologies. In gasification equipment, biomass is transferred to high-grade combustible gas through thermal chemical action at high temperature. The gas can be used for drying, heating, thermal insulation and electricity generation.

Until now, practical biomass gasification equipment takes air as its gasification medium. Despite the low heat value, this kind of gasification stove is characterized by its simple structure and by the fact that it is convenient to operate.

Using gasification equipment, almost all biomass can be transferred to gas fuel which mainly consists of CO, CO2, H2 and CH4. The other part of energy of biomass is used to carry out gasification action. The gasification efficiency of wood is 60% - 80%. Pyrolysis enables the energy recovering rate of rice to reach over 94% and the thermal value of the combustible gas obtained is 2.5* 10 kJ/m³ . The thermal value of combustible gas obtained from cattle manure pyrolyses is 1.7* 104 kJ/m³ . The gasification efficiency of multiple waste is over 80%. The thermal value of combustible gas can be increased by adding hydrogen during pyrolyses processing of biomass.

At the beginning of the 1980's, China started to develop gasification equipment for biomass waste residue that are suitable for various fuel resources. The new gasification equipment has the following characteristics: they can meet various operation requirements on functions, including thermal utilization and power utilization; they are suitable for various fuel resources on features; range of energy output is 20* 10 -80* 104 kW/h and thermal efficiency reaches 75%; fixed bed model is adopted on structure, which is practical, reliable and has a long service life.

Since the 1970's, some European countries begun to study gasification equipment with multiple functions, suitable for different requirements and adopting high temperature pyrolyses technique. There were two kinds of gasification equipment developed in Holland:

- sliding bed gasification chamber

Biomass slides slowly from the top of gasification chamber while gasifying. Oxidizer is flowed upwards from the bottom of gasification chamber and crosses the biomass to gasify it. The temperature of output gas can reach 600 C and there is no tar in the gas.

- sulphurised bed gasification chamber

The ground biomass (with a size of a few mm) is fed to the gasification chamber and gasified while crossing the float material. The gas produced has high temperature that reaches to about 800 C. The sulphurised bed gasifier is mostly suitable for biomass.

In the recent years, great progress has been made in the development of pyrolyses gasification technology of rubbish. Pyrolyses gasification equipment in the USA can treat up to 300 tons of rubbish per day and its gasification efficiency is over 80%. After pyrolyses, rubbish is decomposed to combustible gas, composed mainly of methane and carbon oxide, and a little tar and solid residues that can be used as fuel.

After gasification of the biomass, the combustible gas produced can be used for direct combustion with a relatively low converting efficiency and using catalysis reaction. It also can be transferred to liquid fuel like methane. The combustible gas produced from biomass pyrolyses can be used for electricity generation and cogeneration. There are two kinds of equipment for biomass cogeneration. They are steam turbo combustion turbine and steam jetting combustion turbine with the efficiencies of 35% and 42% respectively.

In a word, the thermal utilization efficiency of biomass is increased greatly by pyrolyses gasification. And by this way, agricultural residue is used efficiently, environmental sanitation is improved, utilization efficiency of natural resources is raised with obvious both environmental and social benefit.

3.2.2 It is a prospective way of biomass gasification technologies to produce biogas from biomass by anaerobic digestion. technology. By this way, the combustible gas is obtained while organic waste is treated and the residue digested can be processed into fodder or fertilizer, which is commonly developed because of obvious economic, environmental and ecological benefits. Some developing countries like China and India are extending and using this technology in rural areas. The family-sized digester technology of China is in the leading position in the world. Up to now, there are 4.75 million small-sized digesters which produce 1.04 billion cubic meters of biogas annually. In addition, all the middle and large scale biogas plants with an electricity capacity of 2077 kW in China can produce 29.1 million cubic meters of biogas annually.

The family-sized digesters in rural China adopt the technique in normal temperature digestion with two kinds of digester models. One is a hydraulic model and the other is a floating cover model. The biogas production rate varies from 0.1 to 0.3 m³ /³ day depending on the temperature in local areas.

On the aspects of treating multiple industry waste water and organic rubbish, some countries adopt highly efficient techniques, such as: anaerobic filter, UASB and sulphurised bed with the digestion temperature at about 36 C3, except for middle and high temperature fully-fixed digestion techniques. France and Japan use and extend high efficiency anaerobic digestion equipment that adopt high density adhering technology to treat organic waste water to international market. Its efficiency is ten times higher than that of traditional methods. Dry digestion technology and two-step anaerobic digestion techniques have been widely researched in recent years, and these can be used to treat solid waste. According to the present technology level, 10 m³ of biogas can be produced from one ton of rubbish, 35 m³ of biogas from one ton of human excrement and urine and 5 - 50 m³ of biogas from one ton of organic waste water with high concentration. In the USA, about 30 billion m³ of biogas are obtained from the treatment of multiple rubbish, waste water and sludge.

3.3 Liquidation of Biomass

The following methods are adopted to transfer biomass into liquid fuels: thermalization, biochemistry, mechanical and chemical method.

Thermalization - gasification, high temperature decomposition and liquidation. The technique whereby synthetic gas gasified from woody fiber is transferred to methanol was carried out at the beginning of this century, but its efficiency reached only 20%. In the chemical industry, a new synthetic technology, that is catalytic methanol composition, is adopted to raise the transferring efficiency from synthetic gas to methanol. Because the purification of synthetic gas and the stability of compositions are very important factors in the transformation, this technology has a high requirement to the synthetic gas after gasified. The technologies of high temperature cracking and liquidation of woody fiber are at research stage. Small scale experiments on wood thermalization are being done in France, which treats about 10 tons of wood each day. Brazil and the USA are also carrying out similar experiments.

The method of biochemistry - hydrolysis and digestion

After hydrolysis, the saccharides in agricultural crops such as sweat sorghum, beet, wheat and sugar cane can be transferred to mixture of water and alcohol by means of digestion and then the alcohol is obtained by separating the mixture. This technique has been used for a long time. In recent years, intermittent digestion is changed to continuous one and the techniques of multiple stage distillation and anaerobic digestion of waste water are adopted. So, the technology that make alcohol by saccharized digestion of biomass gets improved gradually and is considered as an effective way to decrease the production cost of alcohol. Early in 1977, Brazil started to make alcohol from sugarcane and more than 370 factories that annually produced 12 billion liters of alcohol from sugar cane were established. The alcohol was mainly used as fuel for 8 million cars. Because of insufficient domestic petroleum production, the USA also tried to make alcohol from excessive maize to solve the problem of fuel shortage since 1979. More than 100 factories that produced alcohol were set up and each factory made about 100 million liters of alcohol per year on average in the USA. Unlike the manufacturing method used in Brazil, the USA uses the alcohol mixed with gasoline. Holland has mature technology and long-term utilization experience on making alcohol from wheat and beet. The alcohol production cost when taking agricultural crops as raw material is as follows: the cost of raw material is 2/3, the processing cost is 1/6 and the cost of energy consumption takes up 1/6. The process to make alcohol from beet includes: to concentrate beet liquid to syrup, continuous multiple stage digestion, to dry residue, anaerobic digestion of distiller's grains, separation and purification of alcohol and anaerobic purification of waste water, etc. By this processing equipments, 97 liters of alcohol can be produced from per ton of beet.

The mechanical method - pressing and extracting

Now the equipment to process rapeseed oil to diesel substitute is relatively mature. Its process is: to press out oil from rapeseed, esterify rapeseed oil to diesel substitute by using methanol, process rapeseed cake to fodder and collect the byproducts of glycerine and esterification. The main reason why pressed rapeseed oil cannot be used in conventional diesel engine is that the viscosity of rapeseed oil without esterification is different from that of diesel. If rapeseed oil is used directly in diesel engines, excessive carbon will be built up in the jetting nozzle and the combustion chamber after a period of operation.

The chemical method - methanol composition and esterification

In industry, plant oil is esterified to methanol as diesel substitute. Per ton of rapeseed oil can be esterified to 0.05 ton of methanol that can produce about 0.32 tons of diesel substitute.

Compared with fossil fuels, the price of liquid fuel made from biomass is relatively higher now and will be so even in the future. If new breakthroughs can be reached in biomass liquidation technology, using agricultural crops to make alcohol as fuel will be competitive with conventional energy resources in condition of batch production. For this reason, attention has been paid to enzyme hydrolysis technology for woody fiber from abroad in order to replace national acidolysis techniques with microbial digestion as a cheaper solution. This is a kind of biomass liquidation technology has good prospects.

4.1.1 As is well known, the reserves of oil, natural gas and coal are limited, and people worry that fossil energy sources will be exhausted with the continuous increase in their exploitation

4.1.2 Now many countries are aware of the problem of energy transition. The continual increase in the utilization of fossil fuel affects very much the environment and ecology, especially the emission of CO2 which threatens environment and climate more and more seriously. This forces people to consider the dangers involved in relying excessively on fossil fuels. The focus should be turned towards the development and utilization of renewable energy resources

4.1.3 The energy consumption by developing countries has increased four times over the past 30 years and it may continue to increase at this rate. Although this speed is necessary for the developing countries to develop their economies and improve people's living standards, it also results in the shortage of conventional energy resources and continual environmental degradation. Both developing and developed countries will have to pay more and more attention to the exploitation and utilization of biomass energy for the benefit of the whole earth.

4.1.4 Agricultural production in developed countries is excessive. According to one report, the agricultural production of the EEC has exceeded the needs of the food industry. Recently, they raised the question of about 80 million ha. of land being changed to grow energy crops with long life. It is estimated that if per ha. of energy crops with long life can annually produce 20 tons of dried matter with thermal value of 18.5 GJ/ton, then 80 million ha of land can output 30 EJ energy per year. It approaches the total annual present consumption of fossil fuel in the EEC.

4.1.5 Developing countries are actively promoting technical measures helpful for sustainable agriculture development, such as adopting multiple-level utilization technology of matter, realizing "no-waste production", improving resource using efficiency and reaching good circulation of agricultural ecology. The renewing principle of matter circulation is used to make agricultural and by-products, residue and manure as nutrition composition of other animals and plants in order to raise multiple-stage converting efficiency of biomass energy resource.

4.2 Development Trend of Biomass Energy

The development of society raises higher and higher reguirements for energy. Low-grade fuel is no longer suitable for the civilization. That normal solid biomass fuel is converted to high-grade gas and liquid fuel and then further transferred to electricity and hydrogen energy, etc. has became the development trend of biomass utilization.

Along with the development of modern science and technology, the challenge that people will face in the 21st century is how to develop and use, scientifically and reasonably, the biomass energy resource. Looking forward to the future, great biomass energy will accompany people towards a more beautiful developed world.

Reference

1.Research and Development of Biomass Technology in China. China Science and Technology Press. 1992. Page 2-19.